In U.S. Pat. No. 4,252,413 of common assignee herewith, such an electron beam processor is described in which substrate surfaces, as on a web, are passed through a shielded processor, entering at an angle into an inlet or infeed region zone, containing an appropriate collimator system and then passed through a subsequent irradiation treatment, or other processing zone or region, herein sometimes referred to generically as the "curing" region or zone, where electron beam energy is passed through a window of the electron beam generator to impinge upon the surface travelling through the curing or processing zone, and then exiting at an angle in processed state.
Essential to complete curing, for example, of an electron-beam curable or treatable surface being irradiated, is the adequate stripping off of oxygen (or air) layer from the top surface of the substrate before the electron beam irradiates the same in the curing or processing zone. Such oxygen layer, which is inherently carried as a boundary layer with the substrate as it enters the inlet region of the processor, will inhibit the effectiveness and completeness of the electron beam treatment. Oxygen inhibition of free radical initiated polymerization is discussed, for example, in "Radiation Chemistry of Polymeric Systems," A. Chapiro, Inter-Science Publishers, N.Y. (1962), Ch. IV. The inerting of the processor is essential also to eliminate beam-produced ozone and nitrous oxides which can be carried by the product into the work area. Tolerable levels of ozone have been &lt;0.1 ppm, requiring sophisticated gas control techniques for high speed processors as used in crosslinking of film or sterilization applications.
Purging of the oxygen barrier has accordingly been standard procedure, as by introducing pressurized pure nitrogen gas from a liquid nitrogen (LN.sub.2) supply into the processor treatment zones as described, for example, in said patent. The monitoring of the degree of nitrogen purging, however, by location of a sensor at different regions of the processor still does not really determine the remaining oxygen on the substrate surface where the electron beam impinges on the same; and precise information on tolerable oxygen contaminant at such interface for satisfactory cure or other treatment has been difficult.
These problems have become exacerbated as higher speeds of electron-initiated polymerization of coatings, such as inks, polymers and film, are desired and the inerting must keep pace. The high cost of using pure liquid nitrogen purging is another disturbing factor.
There has been no technique readily available, however, adequately to ascertain inerting efficacy. While one can easily determine if the system permits high speed transport of the product with suitable presentation to the processor without pollution of the workplace, as by determining the concentrations of beam-generated pollutants in the work areas and their dependence on line speed time processor current, etc., this is not, for most electron curing applications, the important criterion. The critical test is whether or not the design provides a suitable inerted environment so that an acceptable degree of cure or treatment, again using these terms interchangeably, can be achieved with a modest treatment level (absorbed dose).
What may constitute an acceptable degree of cure, however, depends heavily upon the end application of the product. If, for example, it is a coating which is in direct contact with a consumable product, or a medical product, or if it is rolled into contact with the material surface that eventually is used in direct food contact, the requirements of cure are severe. This type of application must indeed comply with the requirements of the Code of Federal Regulations (Title 21) in force in the United States, wherein materials which can be extracted from the coating are used as a measure of cure quality.
In accordance with the present invention, an analytical technique has been developed for the optimization of system inerting, and in particular has been used to study the effects of nitrogen gas purity and point(s) of injection in the curing process. In view of the great sensitivity of the g.c. assays of degree of conversion, the technique has been used to determined the efficiencies of using "hybrid" inerting, in which relatively economical but lower purity nitrogen (e.g. 99%) is used as an adjunct to the very high purity (99.999%) but more expensive, cryogenically-produced nitrogen. This invention also teaches the significant process efficiencies which are realized using this combined technique, with no diminution in curing efficacy compared with the use of just the purest nitrogen gas.